1. Technical Field
The present disclosure relates to systems and methods for controlling an internal combustion engine to operate in a reduced or variable displacement mode while managing noise, vibration, and harshness (NVH).
2. Background Art
Internal combustion engines are generally most fuel efficient when operating unthrottled at a compression ratio that allows the ignition timing to produce best torque without detonation. To meet customer expectations for acceleration and responsiveness, engines are often sized such that the maximum power exceeds the vehicle's power requirements under most operating conditions. As such, it is necessary to limit the engine's power production to deliver the power expected by the driver. For spark ignition engines, this reduction of engine power is usually done by controlling position of an airflow limiting device, such as a throttle valve, to limit airflow to the combustion chambers. While airflow control is an effective way to control power output, it generally results in reduced fuel efficiency associated with increased pumping losses to move air into the cylinders, combustion heat losses, and reduced cylinder pressures.
A variable displacement engine (VDE) improves fuel efficiency by selectively operating in a reduced displacement mode where one or more cylinders are deactivated to reduce pumping losses and combustion heat losses while increasing the cylinder pressure for the operating cylinders. For optimal fuel efficiency, the minimum number of cylinders operating at maximum power output would be utilized to deliver the requested power or torque. However, operation in one or more reduced displacement modes alters the frequency and magnitude of the torque pulsations or disturbances generated by the cylinder firings, which may be transmitted through the chassis and result in undesirable noise and vibration within the vehicle cabin. In general, cylinder deactivation causes lower frequency and higher amplitude torque pulsations at the crankshaft. As such, operation in the reduced displacement mode is typically limited or constrained to mid-range engine speeds at low or moderate loads. Unfortunately, the constraints that have the most negative impact on the potential fuel economy benefit are associated with unacceptable NVH, including operating at low RPM, idle, and engine lugging, for example. In one study, a fuel efficiency improvement of up to 14% was obtainable using reduced displacement operation without such constraints. Limiting reduced displacement mode operation by imposing an engine lugging limit of 1400 rpm reduced the benefit by 2.1%. Other NVH constraints also adversely impacted the available fuel economy benefit by preventing reduced displacement operation at idle (−2.1%), near idle (such as below 1000 rpm) (−1.4%), in first and second gear (−2.0%), and during engine warm-up (−0.9%). When all constraints (NVH and other) were imposed, the available fuel economy improvement of about 14% was reduced to only about 6%. As such, reducing or eliminating NVH originating from the change in engine firing frequency and magnitude associated with reduced displacement operating modes facilitates increased operating time in these modes and may result in improved overall fuel efficiency for the engine/vehicle.
Prior art approaches to managing NVH in variable displacement engines include limiting variable displacement operation, which negatively impacts the potential fuel economy benefit as previously described, tuning the powertrain mounts or using actively controlled mounts to minimize transmission of vibrations, and active noise cancellation within the vehicle cabin. Other known solutions consist of the addition of one or more counter-rotating elements to reduce or eliminate inertial torque reaction, such as disclosed in U.S. Pat. No. 5,570,615, for example. U.S. Pat. No. 4,163,399 to Yamada discloses a motorcycle power plant having three parallel shafts with the torque converter gear-driven from the crankshaft so the crankshaft and torque converter rotate in opposite directions to provide compactness. Neither the NVH issues associated with operating in a reduced displacement mode nor the NVH issues associated with gear rattle caused by cyclical loads are addressed. Also, there was no expressed intent to match the effective magnitudes of the forward and backward rotating inertias. U.S. Pat. No. 5,282,444 to Ito discloses a powerplant for a personal watercraft that uses a counter-rotating member to improve vehicle stability, but does not address NVH issues, does not eliminate backlash in the gear set, and does not operate in a reduced displacement mode. The watercraft powerplant uses a counter-rotating member that rotates slower than the crankshaft to reduce any yaw moment that would be produced by the gyroscopic effect in response to pitching of the watercraft.
While acceptable for some applications, none of the prior art approaches passively manages NVH associated with the engine firing frequency to expand available operating conditions for a reduced displacement mode using existing powertrain components.